Hitherto, when processing or conveying plastic products or metal plates, the adherend is protected by adhering a surface protection film temporarily so as to prevent the adherend from injuring. As such surface protection film of products, an adhesive film having a pressure-sensitive adhesive layer provided at one side of a base film is used.
Particularly when an optical sheet (optical film) called prism sheet or lens sheet is used as an adherend, a rubber-based pressure-sensitive adhesive is generally used for the surface protection film so as not to optically affect the precisely-formed prism (lens) pattern of acrylic resin (see, for example, Patent Document 1 below). However, since the prism surface has an uneven pattern in the order of several 10 μm, its contact area with the pressure-sensitive adhesive is very small as compared with other adherends. Therefore, it is difficult to stably attach the protection film to the prism face.
For this disadvantage, a technique is proposed in which the pressure-sensitive adhesive layer is made thick so that it can more closely adhere to the uneven pattern of the prism. In this case, however, the pressure-sensitive adhesive is unevenly distributed into the uneven pattern of the prism so that the light transmittance can be made uneven, and as a result, another disadvantage arises in that unevenness of pressing marks (imprinting marks) is produced in the appearance of the protection film attached to the prism face, so that it can be difficult to perform an appearance inspection of the prism sheet together with the protection film attached thereto.
In order to prevent the unevenness in the appearance, a surface protection film with a rough surface (with a large surface roughness) may be attached to the prism sheet. In such a case, however, the surface protection film causes deformation of the prism shape so that dot-like optical defects can be produced in the prism sheet. The dot-like optical defects can degrade the optical properties of the prism sheet and render the product valueless.
However, there is a trade-off between these disadvantages, and in conventional technologies, there has been no surface protection film that can overcome both disadvantages at the same time.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 11-181370